1. Field of Invention
This invention relates to improvements in evaporative pre-cooling devices, and particularly to a device that uses a dual evaporative cooling system to improve the efficiency and reduce electrical demand of packaged rooftop cooling systems frequently used on non-residential buildings.
2. Description of Related Art
A majority of new low-rise non-residential buildings in the United States are cooled with packaged rooftop units (RTU""s). A RTU can include a number of components, e.g., one or more compressors, a condenser section that includes one or more air-cooled condensing coils, condenser fans, an evaporator coil, a supply blower, an intake location for outdoor ventilation air (with or without an xe2x80x9ceconomizerxe2x80x9d to fully cool from outdoor air when possible), optional exhaust air components, and controls. These components are packaged alike by manufacturers to be air cooled. Conventional modes for packaging fail to take advantage of the opportunity to improve efficiency and reduce electrical demand through evaporative condenser cooling. This opportunity is particularly significant in dry climate locations such as in California, where more than 1,000,000 air-cooled RTU""s were operating as of the year 2000.
In climates where summer afternoon temperatures routinely reach 95xc2x0 F. and higher, but with dry air such that wet bulb temperatures rarely exceed 70xc2x0 F., RTU cooling efficiencies can be increased by 20% to 25% using an evaporative condenser air pre-cooler (ECAP). ECAP""s have been available for many years but have not achieved widespread success. Their low sales volume results in part from their added maintenance requirements and in part from the failure of RTU manufacturers to market them. RTU manufacturers have been reluctant to market accessories that appeal to regional markets, especially if those accessories require added maintenance. Strategies that reduce ECAP maintenance requirements and clearly demonstrate favorable economics could substantially enlarge the ECAP market.
High maintenance requirements that have typically plagued ECAP""s result from: hard water deposits, entry of insects and debris, difficult access to key operating components, and biological growth that fouls water feed components. Hard water deposits result from calcium and magnesium in the supply water that concentrate as pure water evaporates. These minerals firmly adhere to piping and evaporative media surfaces, reducing flow rates, clogging water distribution headers, and causing deterioration of the economical rigid evaporative media materials. The entry of bugs and debris also contribute to the disadvantages associated with RTU""s without ECAP""s. Periodic cleaning of condenser coils is required to maintain efficient operation. Evaporative media panel cleaning is complicated by conventional designs. For example, removal of an evaporative media panel is required from the top, where the water distribution header interferes and therefore must also be removed. An ECAP reduces condenser coil cleaning frequency, but its own maintenance requirements are seldom credited for the coil cleaning savings. Biological (algae) growth typically occurs in locations that remain continuously wet, as is true of ECAP sumps. These maintenance issues for traditional ECAP""s offer a clear opportunity to enlarge the market using features that significantly reduce maintenance requirements.
The ECAP market has also been limited by a disconnect between purchasers and maintainers. ECAP""s are usually purchased by management based on a payback analysis prepared by the seller. After installation, the ECAP becomes the responsibility of a maintenance staff or a contractor who seldom pays the energy bill and therefore has little idea of the ECAP value. Monitoring of savings is typically expensive and therefore avoided. As a result, many ECAP""s are removed and considered a failure after a few years in use. This recurring scenario suggests an opportunity for improved designs with economical on-board monitoring and diagnosis electronics.
A major untapped opportunity afforded by RTU design is evaporative pre-cooling of ventilation air. At least 10% of the supply air delivered by RTU""s is typically outdoor air needed for building ventilation. In some cases, particularly for laboratory facilities, RTU""s deliver 100% outdoor air. In warm weather, cooling of ventilation air represents a significant fraction of the total cooling load. In the driest climates, ventilation air can be pre-cooled by the same direct evaporative process used in ECAP""s. However, in most applications an indirect process that adds no moisture to the ventilation air is preferred.
Another opportunity afforded by RTU evaporative pre-cooling is reduction of fan energy consumption. On the condenser side, RTU""s use high airflow rates to compensate for their air-cooled design. And, on the evaporator side, RTU""s typically send indoor air through a contorted path as it is drawn up through return ducts into the RTU, around several tight turns inside the unit, and back down through supply ductwork. The added pressure drop associated with this complex path results in high fan energy consumption that penalizes the system all year, particularly in widely-used constant-speed systems. These high fan speeds are required during peak cooling load conditions. Applying evaporative air pre-cooling to both condenser and evaporator sides allows reduced fan speeds that generate full-year fan energy savings.
In recent years several new RTU pre-cooler products use a non-recirculating water feed system without a pump or a sump. In these lower-cost systems, water from a pressurized source is fed over the evaporative media as needed in response to a moisture sensor at the bottom of the evaporative media. Excess water that reaches the bottom is drained away. These systems have three disadvantages. First, they cannot circulate evaporatively-cooled sump water to a ventilation air pre-cooling coil. Second, they typically use more water than recirculating systems because of their constant drainage. Third, they are more susceptible to fouling with hard water deposits because all hardness minerals are left on the pads of the rigid evaporative media.
These disadvantages suggest a need and opportunity for dual evaporative pre-cooling systems that are capable of fitting both new and existing packaged rooftop cooling units, reducing maintenance requirements, and pre-cooling both condenser and ventilation air, thus facilitating reduced fan operating speeds. In addition, there is a need for a dual evaporative pre-cooling system that can diagnose operation and report energy savings to building owners, operators, electric utilities and any other party responsible for the operation thereof.
The present invention addresses the problems set forth above. The present invention is directed to dual evaporative pre-cooling systems for use as accessories on packaged rooftop cooling units that satisfy the above needs. An exemplary dual evaporative pre-cooling system according to the invention includes one or more evaporative condenser air pre-cooling panels, one or more water sumps, one or more water pumps, an indirect ventilation air pre-cooling coil, a supply pipe from at least one pump to the indirect ventilation air pre-cooling coil, a return pipe from the indirect ventilation air pre-cooling coil to the evaporative condenser air pre-cooling panels, a refill system to replace evaporated water, a motorized valve or second pump to purge and discharge sump water for maintenance purposes, an electrical power supply, and a control system to control and monitor system operations.
Another aspect of the invention is to provide a dual evaporative pre-cooling system for packaged air conditioning units. The dual evaporative pre-cooling system includes an evaporative media disposed in a housing with an air entry side through which incoming air flows. A water distribution device is disposed above the evaporative media. A sump and a pump are located in the housing below the evaporative media. The pump recirculates water through the water distribution device. A ventilation air pre-cooling coil and a plurality of pipes are connected together to allow circulation of a water source discharged from the pump through the ventilation air pre-cooling coil, and to the water distribution device.
According to the invention, each evaporative condenser air pre-cooling panel includes a structural frame, a rigid evaporative media contained within the frame, a water distribution header above the evaporative media, and an inlet screen that prevents insects and debris from entering the system. At least one (primary) evaporative condenser air pre-cooling panel includes a sump disposed below the evaporative media that contains enough water to ensure continuous pump operation without running dry. Other panels may be secondary panels, without sumps, that drain to the sump of the primary evaporative condenser air pre-cooling panel. In an exemplary embodiment, all of the water pumped from at least one of the sumps is delivered through the indirect ventilation air pre-cooling coil before circulating to at least one of the water distribution headers. In alternate exemplary embodiments, a pumped flow of water can be apportioned between the indirect ventilation air pre-cooling coil and at least one of the distribution headers, such that some of the water can bypass the coil and flow directly to the distribution headers, or some of the water can return directly to the sump from the indirect ventilation air pre-cooling coil, bypassing the distribution headers.
In an exemplary embodiment, each distribution header includes of a horizontal pipe perforated with a linear hole pattern. To ensure uniform water distribution on the top of the evaporative media, water is discharged upward from the horizontal perforated pipe against an underside of a semi-cyclindrical distributor surface. The sprayed water ricochets randomly into a widely-dispersed pattern that fully wets the evaporative media.
To prevent freeze-damage, the indirect coil and connecting pipes are designed to drain water back to the sump when the pump is not operating. Drainage is facilitated by a submersible pump with a vertical-axis impeller that delivers water through an upward-sloping pipe to a bottom inlet of a vertical supply manifold in the ventilation air pre-cooling coil. The indirect ventilation air pre-cooling coil can be provided with all horizontal serpentine circuits in parallel. The horizontal serpentine circuits of the ventilation air pre-cooling coil discharge into a vertical return manifold with top outlet. From the indirect ventilation air pre-cooling coil discharge, no xe2x80x9ctrapsxe2x80x9d are permitted before the water emerges from the perforated pipe. When pumped flow stops, air entering the perforated pipe allows water in the indirect ventilation air pre-cooling coil and in the pipes to drain back to the sump. The sump is discharged either by opening a drain valve or activating a pump-out cycle.
In an exemplary embodiment, the water refill system includes: a pressurized water supply line, a solenoid valve, a float switch, and a controller to operate the water refill system. This exemplary embodiment is used in conjunction with controls that limit biological growth by regularly discharging the sump. According to the invention, a control/monitoring system includes the controller (e.g., a microprocessor controller) with a time clock and temperature sensors that detect: an outdoor air, an evaporatively pre-cooled air, a building return air, the sump water, and a return water from the ventilation air precooling coil. Based on at least these five temperature inputs, pre-programmed building operating schedule data, and a cooling demand on the RTU, the controller can decide when to operate the evaporative pre-cooler system to maximize energy savings. The controller also uses this data in conjunction with power monitoring input data to compute and report energy savings, and to diagnose potential operating problems.